This invention relates to a ferrohydrostatic (FHS) separation method and apparatus.
As defined in the specification of U.S. Pat. No. 3,483,969, a ferrofluid is a material comprising a permanent, stable suspension of ferromagnetic material in a suitable liquid carrier. A common ferrofluid comprises fine particles (typically 10.sup.-9 m or less in size) of magnetite in a liquid. In this case, the extremely fine nature of the particles maintains them indefinitely in suspension without sinking or agglomerating.
The use of a ferrofluid to separate materials of different densities, referred to in the art as ferrohydrostatic separation, is also known and is, for instance, described in the specification of U.S. Pat. No. 3,483,969. The materials which are to be separated can be solid particulate materials or liquids which are immiscible with the carrier liquid of the ferrofluid. In essence, the separation process involves applying a magnetic field to the ferrofluid with a view to controlling the apparent density of the ferrofluid within close limits. The materials which are to be separated are then deposited in the ferrofluid, with the result that those materials which have a density exceeding the controlled apparent density of the ferrofluid will sink in the ferrofluid while those which have a density less than that of the ferrofluid will float in the ferrofluid. The sink and float fractions can then be recovered separately.
In all known prior art FHS separators using ferrofluids, the required magnetic field is generated by means of electromagnets or permanent magnets with an iron yoke, with the ferrofluid situated between the pole tips of the magnet. This has a number of significant disadvantages which may be summarised as follows:
1. In order to ensure that the FHS process operates with a well-defined cut point it is essential that the pole tips of the magnet be carefully designed to produce a constant magnetic field gradient in the working space between the pole tips. This can be difficult to achieve even with complicated mathematical models, because of the non-linear magnetic behaviour of iron. As a result it is generally only possible to achieve an approximately constant magnetic field gradient in the ferrofluid. PA1 2. In order to achieve a magnetic field across a suitably large volume to enable the FHS technique to be used for large throughputs, it is necessary to increase the gap between the pole tips of the magnet. This in turn results in an enormous and uneconomical increase in the volumes of iron and copper required to construct the magnet and, in general, in the overall size and mass of the separation apparatus. PA1 3. In the conventional iron yoke magnets the magnetic field strength across the air gap between the yoke tips is non-homogeneous. This means that only a central region of the air gap can usefully be employed in the FHS technique. PA1 1. With a solenoid, it is possible to generate an equivalent magnetic field to that generated by an iron yoke magnet, in the same space, with a far more compact design which requires less iron and copper material. A particularly compact solenoid design is possible if the solenoid is clad with an iron return frame, as mentioned above. PA1 2. Whereas it is necessary with an iron yoke magnet to increase the air gap in order to achieve an increase in throughput of material which is to be separated, with the attendant disadvantages mentioned above, with a solenoid it is possible to increase the throughput merely by increasing the relevant transverse dimension of the solenoid, the axial length of the air gap remaining constant. Because the number of ampereturns required to generate a given magnetic field is dependent on the length of the air gap a solenoid can be scaled up to any required, practical size and still have the number of ampereturns constant. PA1 3. With a solenoid it is possible to design the magnetic field pattern in a simple and highly accurate manner. This facilitates the provision of a magnetic field gradient which is constant, thereby enabling close control to be maintained over the apparent density of the ferrofluid and accordingly over the cut point which is achieved in the FHS separator. As mentioned above, this can, for instance, be achieved by precisely designing the winding of the solenoid, by varying the current density at different positions in the winding or by using a multiple winding arrangement. PA1 4. The magnetic field across the transverse dimension of a solenoid is homogeneous, which means that the same, constant apparent density of ferrofluid can be achieved across the full transverse dimension. Thus the entire transverse dimension can be used for separation and the overall design is accordingly more efficient and compact. PA1 5. Because of the relatively small mass and size of a solenoid compared to an iron-yoke magnet capable of generating an equivalent magnetic field, it is possible to arrange two or more FHS separation units in to provide for multi-stage separation, as described below in more detail.